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These current systems are caused by tidal winds due to solar heating of the atmosphere. Therefore
they are fixed in the Sun-Earth frame and distributed as shown in the global map. As the Earth
rotates under them a weak diurnal variation is seen in the quiet-time ground magnetic field. An
example of the quiet time magnetic field variation at Halley is shown in the figure. It shows the
H, D, and Z components from 0 UT to 24 UT on November 20, 1988. The equivalent currents can be
seen to be consistent with the global SQ current map. The SQ currents have a strong seasonal variation.
They are strongest in summer when solar heating and ionisation of the atmosphere is greatest. This
is demonstrated by comparing the summer (November) example shown in the figure with the winter (
June) example in which the diurnal variation is virtually absent.
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